(i) Field of the Invention
The present invention relates to an LED (light-emitting diode) light source, and more particularly to a readily luminous energy adjustable LED light source for use in an image sensor such as a facsimile, and for use in a LED head such as printer or the like.
ii) Description of the Related Arts
Conventionally, in an image sensor for reading manuscript information such as a facsimile or the like, a plurality of LEDs are used as a light source. Usually, when the LEDs are used as a light source, each LED chip includes a plurality of LEDs connected in series and a plurality of LED chip rows are aligned in parallel to form a line of light source. However, the LEDs vary in the light output due to variance of epitaxial growth when producing the LEDs, and an important consideration is how to keep the luminous energy of each LED chip group uniform.
Conventionally, as shown in FIG. 1, each LED chip row 1 including a plurality of, for example, four LED chips D1 to D4 connected in series is coupled with a current control resistor 2 in series between an anode A and a cathode K, and the LED chips are driven by a constant voltage to keep the luminous energy uniform.
Usually, the resistance of the current control resistor 2 is determined as follows. That is, a wiring pattern is formed on an insulating substrate, and die bonding of an LED chip row 1 is carried out. Then, wire bonding of the LED chip row 1 and the wiring pattern is performed to obtain a series connection of the LED chip row 1. A probe is applied to the LED chip row 1, and a current is applied to the LED chip row 1 in order to emit light. The resistance value of the current control resistor 2 is determined so that the luminous energy of the LED chip row 1 may be optimum. Hence, a chip resistor having the closest resistance value to the optimum resistance is selected and is mounted on to the substrate as the current control resistor 2 in series with the LED chip row 1. However, since the number of different resistance values of the current control resistor 2 is limited, it is difficult to carry out an accurate adjustment of the luminous energy of the LED chip row 1.
In order to overcome this problem, another current control resistance value determination method has been proposed as shown in FIG. 2. In this case, a silicon chip resistor array 3 having a plurality of, for instance, six resistors R1 to R6 with different resistance values, connected in parallel, is coupled in series with the LED chip row in place of the current control resistor 2 shown in FIG. 1. One or more than one of the six resistors R1 to R6 can be selectively bonded so as to obtain the closest resistance value to the optimum resistance, as disclosed in Japanese Utility Model Application No. Hei 1-87590. In this instance, by connecting the silicon chip resistor array 3 in series with the LED chip row 1, the luminous energy of the LED chip row 1 can be quite accurately controlled. However, the luminous energy measurement process, the resistor selection process and resistor bonding process are required, and it takes a long time to carry out the luminous energy measurement process.